Considering Zygardes Color scheme, ability, stats, battle theme, name, and even typing are in Contrast and Comparison to Xerneas and Yveltal, it is more than likely that he is the Z legendary. (Which also means he may get a third form(or forms, think kyurem black and white, but Zygarde X and Zygarde Y instead) or maybe even a mega evolution in the future to help compensate for him not being as strong as Xerneas and Yveltal.)